The present invention relates to an improved process for preparing high molecular weight polyester polymers, and, more particularly, to a process for preparing such high molecular weight polymers from the reaction of alkylene glycols and dicarboxylic acids by first preparing a novel pre-polymer in the form of small generally uniform solid particles which has an intrinsic viscosity in the range of from 0.1 to 0.3 dL/gm and, when subjected to Isothermal Thermogravimetric Analysis, exhibits a thermal weight loss of at least about 1.0 weight percent. The process includes the further step of polymerizing the pre-polymer particles to high molecular weight in the solid state.
High molecular weight polyesters are typically produced commercially according to a melt polycondensation process in which an organic dicarboxylic acid, or a dialkyl ester of the dicarboxylic acid, is reacted with an excess of an alkylene glycol in four stages generally as follows:
(1) a (trans)esterification stage during which at least 95% of the carboxylic acid, or methyl ester, groups are converted to the corresponding hydroxyalkylene ester group; PA1 (2) a vacuum flashing stage wherein a portion of the excess alkylene glycol that was introduced for the reaction in stage (1) is removed; PA1 (3) a pre-polymerization stage dating which more excess alkylene glycol from stage (1) is removed from the reaction mass to yield a low molecular weight poly(alkylene dicarboxylate) pre-polymer; and PA1 (4) a finishing stage during which any alkylene glycol remaining in the reaction mass is removed and high molecular weight polyester is produced. According to this four-stage commercial process, at least one step, usually three steps, and in some cases all four steps, are carried out at reduced pressure to insure that as much excess alkylene glycol as possible is removed from the system. The removal of alkylene glycol is important because its presence during the finishing step of a conventional process can interfere with further polymerization and the formation of a high molecular weight polymer product. PA1 (A) forming a low molecular weight solid pre-polymer particle by reacting at least one alkylene glycol and at least one di- or tricarboxylic acid according to the steps of: PA1 (B) polymerizing the isolated particles in the solid state. PA1 (a) remaining acid (ester) groups are (trans)esterified; PA1 (b) excess glycol is removed from the pre-polymer; and PA1 (c) a high molecular weight polymer is produced substantially simultaneously. PA1 [1] dissolving 1.0.+-.0.2 grams of the poly(alkylene terephthalate) in 25 ml nitrobenzene (dried over molecular sieves) at 150.degree. C., PA1 [2] cooling the solution to room temperature and adding 25 ml chloroform, 10 ml methanol and 1 ml of a 20% by weight solution of lithium chloride in methanol, and PA1 [3] titrating the solution to an endpoint using approximately 0.1N sodium hydroxide in benzyl alcohol. A blank is obtained by repeating the above titration with all of the ingredients except the poly(alkylene terephthalate). The carboxylic acid group concentration is calculated using the formula: ##EQU1## PA1 [1] approximately 1.0.+-.0.1 gram of the poly(alkylene terephthalate) and 1.0 gm succinnic anhydride are dissolved in 25 ml nitrobenzene (dried over molecular sieves), at 150.degree. C.; PA1 [2] the mixture is maintained at 150.degree. C. for 4 hours; PA1 [3] cooled to room temperature; PA1 [4] 50 ml methanol is added to ensure complete precipitation of the polymer; PA1 [5] the precipitate is filtered and washed two times with fresh methanol; and PA1 [6] dried in vacuum at 100.degree. C. for 12 hours. The total carboxylic acid group concentration is then determined by the above titration method. PA1 [1] equilibrate at 35.degree. C.; PA1 [2] ramp 200.degree. C./min to 210.degree. C..+-.5.degree. C.; and PA1 [3] isothermal at 210.degree. C. for 1000 minutes. The maximum percent weight loss at the end of 1000 minutes is recorded as the "isothermal thermal weight loss" for the poly(alkylene terephthalate) particle.
According to the commercial process described above, alkylene glycol is typically introduced into the (trans)esterification step at levels which are at least from 2 to 3 times, and in some instances up to from 5 to 10 times, the level required to insure a high conversion, e.g., normally at least &gt;95%, of the acid (ester) groups in a minimum of time and at as low a temperature as possible. Generally higher levels of excess alkylene glycol are required when a dicarboxylic acid is used as a reactant in stage (1) than when a dicarboxylic diester is used as a starting reactant.
In addition to conducting the process at reduced pressure, the process, when maintained at elevated temperatures for extended periods of time for the purpose of increasing the molecular weight of the reaction product, can also result in the formation of undesirable by-products. For example, a reaction mixture comprising terephthalic acid, ethylene glycol, antimony oxide (as catalyst) and poly(ethylene terephthalate) held at elevated temperature for an extended period of time can result in the formation of acetaldehyde as a contaminant, and a reaction mixture of terephthalic acid, 1,4-butanediol, tetrabutyl titanate (as catalyst) and poly(butylene terephthalate) held at a temperature in the range of 230.degree. C. for an extended period of time can result in the conversion of the 1,4-butanediol to tetrahydrofuran as an undesirable by-product.
From as early as 1939, it has been known that it is possible to increase the degree of polymerization of certain solid condensation polymers by heating (but not melting them) in an inert gas atmosphere. The phenomenon has been called solid state polymerization, polymer build-up and solid state polycondensation. Increasing the molecular weight of poly(butylene terephthalate) (PBT) by solid state polymerization can be accomplished when the starting PBT pre-polymer, prepared according to a conventional multi-step melt polycondensation process of the type described hereinabove, has an intrinsic viscosity in the range of from 0.5 to 0.7 dL/gm, and a majority, i.e., at least about 95%, of the end groups are in the form of carboxylic esters. However, as the starting intrinsic viscosity decreases below 0.3 dL/gm, the PBT pre-polymer becomes increasingly more difficult to solid-state polymerize to high molecular weight. As described by F. Pilati, et. al. in "A Model Description for Poly(Butylene Terephthalate) Solid-State Polycondensation", Polymer Process Engineering, 4(2-4), 303-319 (1986), the teachings of which are incorporated herein by reference, only by reducing PBT particle size to a powder and increasing the ratio of hydroxyl to carboxylic acid end groups, can one hope to achieve intrinsic viscosities greater than 1.0. However, carrying out solid-state polymerization of a powder is impractical, and it cannot be employed with success on a commercial scale.
The wide use of polyester polymers in fibers, molding resins, films, coatings and the like create a need for an improved process for more efficiently preparing high molecular weight polyester polymers using fewer steps, at lower temperatures, without the risk of forming undesirable by-products, and without the need for substantial vacuum.